Hydroxyaromatic and alkoxyaromatic aldehydes are very important products, which are used as flavors and fragrances and as intermediates in numerous fields, such as, for example, agrochemicals, pharmacy, cosmetology, and other industries.
The ortho- and para-hydroxybenzaldehydes, 4-hydroxy-3-methoxybenzaldehyde and 3-ethoxy-4-hydroxybenzaldehyde, named “vanillin” and “ethylvanillin” respectively, are among the most important products.
Various processes have been proposed for the synthesis of aromatic aldehydes.
The most important processes are based on the functionalization of a phenolic starting compound, phenol, catechol derivative, guaiacol (or 2-methoxyphenol), guaethol (or 2-ethoxyphenol).
In this type of process, the phenolic compound is generally involved in a salified form, for example, in the form of a sodium salt.
Thus, for example, numerous processes for preparing vanillin involve a guaiacol salt as substrate, to which is then added a formyl group, in the position para to the hydroxyl group, by various methods.
One conventional route to vanillin involves a condensation reaction of glyoxylic acid with guaiacol, in basic medium, to give 4-hydroxy-3-methoxymandelic acid. This product is then oxidized to produce vanillin.
The reaction is commonly conducted in the presence of sodium hydroxide and with an excess of guaiacol, with glyoxylic acid being the deficit reactant.
Thus, at the end of the condensation reaction, an aqueous reaction mixture is obtained that comprises the sodium salt of 4-hydroxy-3-methoxymandelic acid, the precursor to vanillin, secondary products, such as the sodium salts of 2-hydroxy-3-methoxymandelic acid and 4-hydroxy-5-methoxy-1,3-dimandelic acid, and a greater or lesser excess of sodium guaiacolate.
In this reaction mixture, therefore, there are a number of types of salified phenolic compounds present, namely guaiacol in excess in the form of sodium guaiacolate, and the products of the reaction which are also salified phenolic compounds, such as the sodium salts of 4-hydroxy-3-methoxymandelic acid, 2-hydroxy-3-methoxymandelic acid, and 4-hydroxy-5-methoxy-1,3-dimandelic acid.
For economic reasons it is important to recover the unreacted starting substrate. However, the operation is not easy, since the guaiacol is in the form of sodium guaiacolate and is present alongside phenolic compounds which are also salified and have a closely related structure.
In certain processes described in the prior art, especially in FR 2 132 364, the sodium guaiacolate, at the end of the condensation reaction, is converted to guaiacol by an acid treatment, most often with sulfuric acid.
The unconverted guaiacol is then extracted from the acid solution by an extraction treatment using a hydrocarbon, for example, benzene or toluene.
The drawback of a method of this kind is that it employs an organic solvent, thereby giving rise to additional distillation operations in order to be able to recycle the organic solvent and the substrate recovered. Moreover, in the course of the distillation, there are secondary reactions which lead to the formation of heavy products.
Furthermore, the neutralization of sodium guaiacolate with sulfuric acid produces sodium sulfate, leading to the formation of substantial salt effluents.
Moreover, the guaiacol recovered must be salified again in order to be introduced into the condensation reaction with glyoxylic acid.
Similarly, the reaction mixture, comprising the mandelic compounds with a free hydroxyl group, must be salified again in order to be introduced into the oxidation reaction that allows vanillin to be obtained.
In order to overcome these drawbacks, the invention provides a method that allows the excess of phenolic starting compound in salified form, especially sodium guaiacolate, to be recovered, by a method which does not involve this step of neutralizing sodium guaiacolate to guaiacol, with the attendant need for said guaiacol to be extracted using an organic solvent which must subsequently be separated by distillation.